1. Field of the Invention
This invention relates generally to the process vessels having internal partitions. More specifically this invention relates to an apparatus and method for flushing an isolated end closure while balancing pressure across an internal partition that isolates the end closure.
2. Description of the Prior Art
Pressure vessels that contain particulate material for contacting fluids such as gas or liquid process streams are standard features of the chemical and refining industries. Internal partitions often subdivide the interior of the a pressure vessel into different chambers to permit staged or multiple contacting operations within a single pressure vessel. These partitions routinely take the form of or are used in conjunction with collection or distribution grids. Process requirements, such as the collection and distribution of fluids, regularly dictate the employment of relatively flat partitions.
As pressure vessels become large, the need for flat partitions create a number of mechanical problems. A differential pressure across a flat partition of as little as 2 psi or less can cause structural damage across the partition. Structural damage to the partitions has the potential to create leaks across the partition or in associated distribution/collection piping. Such leaks typically contaminate the zones created by the partition. As a result those designing partitions or grids seek to balance pressure across grids or partitions.
Aside from partition loads, pressure in large vessels affects the vessel configuration. Concave end closures or "heads" present at the ends of the pressure vessel provide the most practical means for closing the ends of a large vessel. Full diameter flat partitions cannot fit into the rounded head. The incompatible geometry between the rounded head and the flat partitions prevent utilization of the head volume for process purposes. Nevertheless, maintaining structural integrity of the end grid demands pressure balancing between the head volume and the adjoining volume on the opposite side of the partition. In addition, the volume of the head represents a non-process or dead volume, i.e. a volume excluded from the normal process flow stream. Dead volumes can lead to vessel or process deterioration from corrosion, condensation, solidification or contamination.
Dead volumes ordinarily receive a purge or flush stream to keep the area active and avoid the aforementioned problems. The volume of the head serves as an equalization chamber. A small opening or port in the partition communicates fluid from the equalization chamber into the operational process chamber on the opposite side of the partition. In many processes the channeling of fluid from the equalization chamber to a process chamber hinders process performance.
A simulated moving bed adsorbent process exemplifies a process that regularly uses multiple partitions in relatively large pressure vessels. U.S. Pat. No. 2,985,589, the contents of which are hereby incorporated by reference, describes the moving bed adsorbent process in detail. The process distributes and collects process streams from multiple chambers of adsorbent defined by internal partitions located within a pressure vessel and arranged as distribution/collection grids. Periodic shifting of the input and effluent streams over the chambers simulates movement of the adsorbent and permits delivery or withdrawal of the streams with a desired concentration profile. Delivering or withdrawing the streams requires flat distribution grids.
Flat distribution grids and concave end closure heads in the simulated moving bed process again result in a non-process volume above or below the end grids in the vertical adsorbtion chamber. A small flow of a flush fluid, usually comprising a desorbent material, flushes the head and passes out of the head through a grid opening into the adjacent adsorbent bed. Proper selection of the flush fluid ordinarily prevents contamination of the streams entering or leaving the adjacent bed. However, leakage of a process stream into the head volume can cause the flush fluid to carry contaminants into the bed.
In the absence of leakage, the addition of the flush fluid can still reduce maximum recovery from the adsorbent bed and interfere with the maximization of purity and recovery. The addition of desorbent to the adsorbent bed can reduce recovery by taking up adsorbent capacity. Maintaining a low flush flow rate causes circulation of fluid between the equalization and process chamber as pressure fluctuations within the pressure vessel pump fluids across the grid opening. Circulation of fluid between equalization and process chambers can contaminate the process chamber.
More importantly, the addition of desorbent hinders an accurate accounting of flow through the adsorbent beds. In the usual case of a liquid system, one effluent stream from the system floats on pressure control while flow controllers regulate the flow rate of the other input and effluent streams. The fast pressure response time of a liquid full system renders a complete flow control of all input and output stream impractical. Thus as several head flush stream enter the system, the cumulative effect of these streams throws off the fluid flow calculation for a given zone which tends to move the process away from optimum conditions. The common use of multiple vessels to contain multiple the absorbent beds and the resulting additional head flushes amplifies the difficulty of maintaining peak flow conditions.
It is an object of this invention to provide a pressure vessel capable of operating with flat partitions or grids that reduces or eliminates the problem associated with a head flush.
It is a yet further object of this invention to improve the operation of simulated moving bed adsorption process.